Fix fs.write signature to use string overload instead of Buffer

[Copilot]

Addresses review feedback on incorrect fs.write usage in Writer/File.ts.

Changes

• Remove unnecessary Buffer conversion: Pass string directly instead of Buffer.from(string)
• Use string overload signature: fs.write(fd, string, position, encoding) instead of fs.write(fd, buffer, offset, length, position)
• Fix position parameter: Use null to append at current file position instead of 0 which overwrites

// Before
const content = Buffer.from(buffer.reduce((r, item) => r + item, ""))
const r = await fs.write(this.descriptor, content, 0, content.length)

// After
const content = buffer.reduce((r, item) => r + item, "")
const r = await fs.write(this.descriptor, content, null, "utf8")

The position 0 was causing each write to overwrite the file from the beginning, breaking multiline writes.

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